Many endogenous peptides have been described as key components of biological processes. Some of these peptides have been identified as key therapeutic agents for the management of various disorders. In general, endogenous peptides are more desirable as therapeutic agents than synthetic peptides with non-native sequences, because they do not produce an immune response due to their endogenous character. In addition, endogenous peptides are highly specific for their target receptors and are easy to synthesize and manufacture. However, a major difficulty with the delivery of such therapeutic peptides is their short plasma half-life, mainly due to rapid serum clearance and proteolytic degradation via the action of peptidases.
Peptidases break a peptide bond in peptides by inserting a water molecule across the bond. Generally, most peptides are broken down by peptidases in the body in a manner of a few minutes or less. In addition, some peptidases are specific for certain types of peptides, making their degradation even more rapid. Thus, if a peptide is used as a therapeutic agent, its activity is generally reduced as the peptide quickly degrades in the body due to the action of peptidases.
One way to overcome this disadvantage is to administer large dosages of the therapeutic peptide of interest to the patient so that even if some of the peptide is degraded, enough remains to be therapeutically effective. However, this method is quite uncomfortable for the patient. Since most therapeutic peptides cannot be administered orally, the therapeutic peptide would have to be either constantly infused, frequently administered by intravenous injections, or administered frequently by the inconvenient route of subcutaneous injections. The need for frequent administration also results in many potential peptide therapeutics having an unacceptably high projected cost per treatment course. The presence of large amounts of degraded peptide may also generate undesired side effects.
Discomfort in administration and high costs are two reasons why most therapeutic peptides with attractive bioactivity profiles are not developed as drug candidates. Instead, these therapeutic peptides are used as templates for the development of peptidomimetic compounds to substitute for the therapeutic peptide. Biotechnology and large pharmaceutical firms frequently undertake lengthy and expensive optimization programs to attempt to develop non-peptide, organic compounds which mimic the activity seen with therapeutic peptides without incurring an unacceptable side effect profile. For example, cyclic peptides, peptidomimetics and small molecules coming from expensive SAR (Structure Activity Relationship) and molecular modeling studies have led to the development of an incredible amount of peptide mimics. However, these peptide mimics in no way reflect the exact original biological nature of the therapeutic peptide, and thus are inferior to the endogenous therapeutic peptide as therapeutic agents.
An alternative to creating peptide mimics is to block the action of peptidases to prevent degradation of the therapeutic peptide or to modify the therapeutic peptides in such a way that their degradation is slowed down while still maintaining biological activity. Such methods include conjugation with polymeric materials such as dextrans, polyvinyl pyrrolidones, glycopeptides, polyethylene glycol and polyamino acids, conjugation with adroitin sulfates, as well as conjugation with polysaccharides, low molecular weight compounds such as aminolethicin, fatty acids, vitamin B12, and glycosides. These conjugates, however, are still often susceptible to protease activity. In addition, the therapeutic activity of these peptides is often reduced by the addition of the polymeric material. Finally, there is the risk of the conjugates generating an immune response when the material is injected in vivo. Several methods include ex vivo conjugation with carrier proteins, resulting in the production of randomized conjugates. Since conjugates are difficult to manufacture, and their interest is limited by commercial availability of the carriers, as well as by their poor pharmaco economics.
There is thus a need for novel methods to modify therapeutic peptides to protect them from peptidase activity and to provide longer duration of action in vivo, while maintaining low toxicity yet retaining the therapeutic advantages of the modified peptides.